1. Field
The disclosure herein relates to an inducer employed to increase the pressure of a liquid introduced to a pump. More particularly, the inducer increases the pressure of a liquid propellant being pumped to the combustion chamber of a rocket engine.
2. Description of the Related Art
Liquid fuel rocket engines typically include tanks containing a liquid oxidizer, such as liquid oxygen, and a liquid fuel, such as liquid hydrogen, collectively called propellants. To reduce the thickness of the tanks and their associated weight, the liquid propellants are usually at a relatively low pressure. The liquid propellants are pumped to a combustion chamber and then ignited to generate thrust. To achieve sufficient thrust, the liquid propellants must be sufficiently pressurized prior to introduction to the combustion chamber. A pump, such as a turbopump, is used to pressurize the liquid propellants. As a first step, an inducer may be located between the propellant tanks and the turbopump to produce an initial pressure increase.
The inducer is an axial flow pumping device, typically a first element of low weight, high performance pumps, for use in liquid propulsion rocket engines. The inducer receives the liquid propellant at a very low inlet pressure and provides sufficient discharge pressure for the next pump stage, usually a radial impeller, to operate safely at high shaft speeds. The inducer must achieve satisfactory discharge pressure at the inducer exit with extremely low pressure at the inlet. The inducer includes a number of rapidly spinning blades to draw the liquid propellant through the inducer. Vortices tend to form on the tips of the blades causing cavitation damage to the blades and a variety of vibrations associated with vortex cavitation instabilities that may be detrimental to the engine operability or life.
It is known that enclosing the tips of the blades in a shroud eliminates the formation of vortices. For example, U.S. Pat. No. 4,642,023 entitled “Vented Shrouded Inducer,” discloses a shroud with a series of holes that allows counterflowing fluid around the outside of the shroud to flow back into the impeller. U.S. Pat. No. 7,070,388, entitled “Inducer with Shrouded Rotor for High Speed Applications,” discloses an inducer rotor with rotor blades terminating at a shroud. The shroud has a variable thickness both to reduce weight and to maintain a uniform gap between the shroud and a housing wall during high speed rotation. U.S. Pat. No. 7,931,441, entitled “Inducer with Tip Shroud and Turbine Blades,” discloses an inducer with two sets of blades arranged axially one after the other. The upstream blades are full size and the downstream blades are half size. The downstream half size blade tips are enclosed in a shroud.
U.S. Pat. Nos. 4,642,023; 7,070,388 and 7,931,441 are incorporated by reference herein in their entireties.
At high rotating speeds, even shrouded blades are subject to a manifestation of cavitation-related hydrodynamic phenomena, including the type referred to as alternate blade cavitation. Alternate blade cavitation manifests as long and short vapor cavities on alternate blades. When there are only two blades, alternate blade cavitation is inherently asymmetric, with a short cavity on one blade and a long cavity on the other blade, resulting in radial load imbalance. A symmetric pattern is only possible with an even number of at least four inducer blades (e.g. 6 or 8 blades also achieve symmetry). Four bladed inducers are utilized in many present day rocket engines. In a four bladed inducer, alternate blade cavitation is characterized by a stable pattern of two large cavities on one opposing pair of blades and two small cavities on the other pair of opposing blades. Such stable patterns result in low radial loads beneficial to bearing life and do not generate traveling pressure instabilities or adverse system vibrations.
However, ultra high suction capability requires a very low inlet flow coefficient (very low ratio of axial inlet velocity to blade tip speed), which in turn requires very low blade angles with respect to the tangential direction, producing a high degree of fluid flow blockage. Higher blade counts exacerbate the blockage problem at low blade angles.
There remains a need for an inducer having the fluid flow capacity of a two or three blade configuration and the cavitation stability of a four blade configuration.